My full beam keeps switching on automatically. Also lights have started to not turn off when I have switched them off. Two days ago my battery was dead after being left overnight (RAC said lights had drained it) even though ignition key was removed. Today my horn sounded after I had parked until I switched the engine off. Relay's checked by mechanic but has not solved issue, mechanic somewhat confused!

I'd say either a fault board at the steering wheel or a fuse box.